As part of the Libor investigation, the U.S. Department of Justice have handed subpoenas to several big banks in the U.S. In Japan, officials are investigating Tibor - Tokyo interbank offered rate.

Some banks may already be off the hook - UBS said the Justice Department granted them "conditional leniency or conditional immunity" for participating in the investigation after they were subpoenaed for Libor manipulation earlier this year. Barclay's is also reportedly cooperating in the investgiation, as the bank noted in its filings this year, according to the WSJ.

Euribor is set by 44 banks, compared to 15 banks that set Libor. This makes the rate harder to manipulate and could weaken the European Commission's case.